It is known in immunoassays that the labeled indicator bound or complexed with the antigen analyte must be separated from those that are unbound or "free". When conducting the assay in a slide test element, it is conventional to achieve such bound-free separation by applying a solvent stream, such as water, to the center of the test element while the applicator and test element remain stationary with respect to each other. E.g., in U.S. Pat. No. 4,517,288 the technique is to apply "a stream of a solvent, in which the labeled indicator is soluble, . . . to substantially the center of the reaction zone," col. 5. That there is no relative movement between application and test element is apparent from the fact that "the solvent migrates radially out from the center . . . [and] unbound reactants . . . if visible, would appear as a ring around the reaction zone . . . , ibid.
Such a stationary technique has continued to be the method of choice even in more modern assays, as is apparent from, e.g., the analyzer described in FIG. 11 of U.S. Pat. No. 5,174,960.
I have discovered that such a process of separating bound labeled indicator from free is unsatisfactory, because, in some test elements at least, it tends to leave unseparated free indicators within the center of the washed zone, the same area that is scanned for detection. That is, a staticly disposed stream will leave relatively untouched, the free indicators directly under the center of the impinging stream. The cause of this is the lack of a sufficient fluid flow at the static center, unlike the periphery, effective to sweep the free labeled indicator through the test element. This is because the pool of liquid intersecting a typical spreading layer of a dried test element, will flow rapidly into that layer only around the intersect edge formed by the meniscus of the pool and the surface of the spread layer. Very little flow velocity is experienced under the center of the stream. This is particularly apparent when the sample fluid initially deposited on the surface layer of the porous test element is of higher viscosity levels due to protein, lipids, etc., than the wash liquid, making it more difficult to move or transport the free indicators. Dilution of the sample could reduce this effect, except that dilution of the sample has its own disadvantages, e.g., it reduces concentration of analyte and therefore degrades signal to noise, and adds potential error from the dilution step.
Yet another drawback of a fixed stream is, that it can physically wash away the top layer of the element, such as if that layer is a blush polymer spreading layer.